Efficiency of splitting water with sunlight reaches new heights

Hydrogen is often touted as an environmentally-friendly fuel - but the gas is only as clean as the method used to make it. Now, however, scientists have invented a solar-powered method for splitting water which they claim is the most efficient to date1.

Michael Gr?tzel and colleagues from the Federal Polytechnic School of Lausanne, Switzerland, hope that their method could one day provide a cheap and efficient technology to produce abundant hydrogen using the Sun’s rays. 

When water is split into its component elements, hydrogen is produced at the cathode of an electrochemical cell, while oxygen bubbles from the anode. It is this latter half that Gr?tzel’s team have focused on, using a new kind of photoanode made from the iron oxide haematite (Fe2O3).

’Iron oxide has been considered as water splitting catalyst for more than 30 years,’ said Gr?tzel. ’However, it is notorious for its very low efficiency in bringing about the photooxidation of water to oxygen.’

His team has increased that low efficiency by boosting the surface area of the material. It is rough enough to expose 20 times more surface than a completely smooth electrode of the same size. Using structural analyses, the researchers showed that this phenomenon can be attributed to the branched nanoscale structure of the haematite , which they formed by chemical vapour deposition at atmospheric pressure (APCVD) using iron pentacarbonyl, Fe(CO)5, as a precursor. 

They also found that a dash of silicon, added during the vapour deposition process, helps the electrical charge generated when light hits the haematite to travel easily through the material. Finally, modifying the hematite surface with cobalt significantly improved the performance of the electrode in photooxidation experiments. 

Copying nature

Under both natural sunlight and artificial lights, Gr?tzel’s team found that they converted the energy of incoming photons of light into electrical current with up to 42 per cent efficiency, which they describe as ’unprecedented’. Previous cells, using more complicated anodes, have achieved up to 37 per cent. 

The scientists believe that their system mimics photosynthesis by ensuring that several photons can work together to create the two electrons needed to oxidise each atom of oxygen in water into its elemental form.

Nathan Lewis, a photochemist at the California Institute of Technology, Pasadena, told Chemistry World  that ’the work represents an interesting extension of nanostructured photoelectrodes to iron oxide, a potentially very cheap electrode material.’

’But several significant advances in the quantum yield, kinetics, efficiency and overlap with the solar spectrum of this system still need to be made before this material is of practical value,’ he cautioned.

Guildford-based company Hydrogen Solar has just received funding from the UK Department for trade and industry (DTI) for an 18-month program that will include collaboration with Gr?tzel’s laboratory. The company’s CEO Brian Holcroft commented:

’[This research] highlights some potentially important signposts into understanding the operation of these materials. There is clearly more work to do here, but this offers valuable insights to build upon.’

What about the cathode?

Meanwhile, in an independent effort, researchers from Japan, China, and the UK have developed a new way of using light to trigger the other side of water splitting, namely the reduction of hydrogen in water to its elemental form2. They coupled a genetically-engineered version of human serum albumin protein with a zinc compound based on porphyrin, the molecule which is used to carry oxygen around the body.

The molecular complex could capture solar energy and use it to generate hydrogen gas, with 25-30 per cent efficiency.

’It’s very exciting to prove that we can use these biological structures as a conduit to harness solar energy,’ said Stephen Curry of Imperial College, London, and part of the team. ’In the long term, these synthetic molecules may provide a more environmentally-friendly way of producing hydrogen.’

Michael Gross